Fixed issue with ReadPreference tests

There was an issue with the tests not properly detecting that read
options were actually readPreference objects. I believe this is because
the readPreference objects were now imported out of mquery, so I just
setup mquery to export the reference to the readPreference it uses.
fixed a small typo left from debugging
fixed up a doc string and deleted a duplicate test
aded unsafe writes and fixed up sorting to work with old format
Added comments & fixed tests to work

Added back a specific test on sort(), which was mistakenly removed.
Also, updated the populate tests to use the new, supported syntax for
sorting.